In recent years, a microcomputer provided in an in-vehicle control apparatus needs to receive, as an input, a plurality of voltages such as a voltage for a core voltage and a voltage for IO. Therefore, a rise sequence and a falling sequence for voltage supply are defined in advance. Further, the microcomputer generates an internal voltage in order to write data into a storage device such as a FLASH memory included in the microcomputer. In a case where the microcomputer is fallen, it is necessary to keep supplying a voltage to the microcomputer until this falling is completed so that the microcomputer keeps generating the internal voltage until the falling is completed.
In a case where supply of electric power from an in-vehicle power source (such as a battery) is interrupted, the microcomputer for controlling an in-vehicle apparatus stops arithmetic processing and writes an arithmetic result until then into a memory. In the in-vehicle power supply apparatus, in order to supply a rated voltage required by the microcomputer during a period in which the microcomputer writes the arithmetic result into the memory and until the internal voltage in the microcomputer is discharged, a capacitor is connected with an input terminal of the microcomputer. Note that a phenomenon in which supply of electric power is interrupted by the in-vehicle power source occurs in a case where, for example, a voltage of the in-vehicle power source is rapidly reduced by starting a load that consumes large electric power or by other reasons, wiring is disconnected, and relay controlled in response to an operation of an ignition switch or the like is turned off for some reason.
In a case where interruption of the relay caused by the above factor, disconnection of the wiring, and the like occur, when the in-vehicle power supply apparatus detects that a voltage supplied to the microcomputer is reduced, the in-vehicle power supply apparatus transmits a reset signal to the microcomputer. Then, the microcomputer stops the arithmetic processing in accordance with the reset signal and writes the arithmetic result into the memory, thereby entering into a standby state. Until the microcomputer enters into the standby state, the in-vehicle power supply apparatus needs to keep supplying a predetermined operation voltage and a predetermined current to the microcomputer. In order to cause the microcomputer to securely implement the above operation, it is necessary to increase a capacity of the capacitor connected with the input terminal of the microcomputer to ensure supply of electric power. This increases costs of the in-vehicle power supply apparatus.
As a method for solving the problem, PTL 1 below monitors an intermediate voltage V2 obtained by reducing a power supply voltage V1 and detects reduction of the voltage. Thus, reset response of the microcomputer is improved, as compared to a case where a microcomputer voltage is monitored, so that an unstable operation of the microcomputer is avoided.
Meanwhile, because a voltage of the microcomputer has been reduced in recent years, an internal core voltage becomes 3.3 V and a consumption current tends to be increased. Meanwhile, a voltage of 5 V, which has been conventionally used, is increasingly used only for an AD port and an I/O port, and therefore a consumption current tends to be decreased. This voltage of 5 V is also supplied to a control IC (Integrated Circuit) and the like other than the microcomputer.
The control IC other than the microcomputer has a factor of a noise source, and therefore, it is necessary to include an electrolytic capacitor as a measure against noises, which is a different reason from the reason for including the above capacitor for supplying electric power to the microcomputer. Specification of the control IC variously changes depending on an environment in which the control IC is mounted and further the microcomputer needs various voltage levels as described above, so that it is difficult to unconditionally determine how much capacities are enough for the capacitors. Also in view of this, it is necessary to ensure safety by increasing she capacity of the capacitor connected with the input terminal of the microcomputer.